Just enough salt is used to prevent the raw fish from rotting (chemical decomposition). A fermentation process of at least six months gives the lightly-salted fish its characteristic strong smell and somewhat acidic taste.

When a can of surströmming is opened, the contents release a strong and sometimes overwhelming odour. The dish is ordinarily eaten outdoors. According to a Japanese study, a newly opened can of surströmming has one of the most putrid food smells in the world, even more so than similarly fermented fish dishes such as the Korean Hongeohoe or Japanese Kusaya.[1]

The Baltic herring, known as strömming in Swedish, is smaller than the Atlantic herring, found in the North Sea. Traditionally, the definition of strömming is herring fished in the brackish waters of the Baltic north of the Kalmar Strait.[2] The herring used for surströmming are caught just prior to spawning.

At the end of the 1940s, producers lobbied for a Royal ordinance (Swedish: förordning) that would prevent incompletely fermented fish from being sold. The decree forbade sales of the current year's production in Sweden prior to the third Thursday in August. While the ordinance is no longer on the books, the retailers still maintain the date for the "premiere".[3]

Surströmming, fermented herring served on flatbread with boiled potatoes and salad

Fermented fish is an old staple in European cuisines. The oldest archeological findings of fish fermentation are 9200 years old and from the south of today's Sweden.[4] More recent examples are the ancient Greeks and Romans that made a sauce from fermented fish called garum,[5] and Worcestershire sauce also has a fermented fish ingredient.

One explanation for the origins of this method of preservation is that it began long ago when brining food was expensive due to the cost of salt.[6] The fish are marinated in a strong brine solution that draws out the blood, which is replaced by a weaker brine, and the fermentation is done in barrels prior to canning.

During the 17th century, surströmming was supplied as army rations in the Thirty Years' War. Swedish soldiers who did not come from the area where this was staple food, as well as foreign conscripts, refused to eat it.[citation needed]

The canning procedure, introduced in the 19th century, enabled the product to be marketed in shops and stored at home, whereas at one time the final stage would have been storage in large wooden barrels and smaller, one-litre kegs. Canning also enabled the product to be marketed farther south in Sweden to homesick northern Swedes and to southern Swedes as a curiosity and party food, serving as a background to schnapps as other spicy herring preparations do.[citation needed]

Historically, other fatty fish such as salmon and whitefish have been fermented in a way not unlike surströmming; the original gravlax resembled surströmming, whereas nowadays gravlax is made by covering the fish with a salt and sugar mixture that permeates the fish so that it is pickled without the type of fermentation used for surströmming occurring.[citation needed]

The fermentation starts from a lactic acid enzyme in the spine of the fish, and so the fermentation is by autolysis; together with bacteria, pungent smelling acids are formed in the fish such as propionic acid, butyric acid and acetic acid. Hydrogen sulfide is also produced. The salt raises the osmotic pressure of the brine above the zone where bacteria responsible for rotting (decomposition of proteins) can thrive and prevents decomposition of fish proteins into oligopeptides and amino acids. Instead the osmotic conditions enable Haloanaerobium bacteria to prosper and decompose the fish glycogen into organic acids, making it sour (acidic).[citation needed]

The herring are caught in April and May, when they are in prime condition and just about to spawn. Prior to spawning, the herring have not fattened. They are then put into a strong brine for about 20 hours which draws out the blood, the heads are removed and they are gutted and put into a weaker brine solution. The barrels are then placed in a temperature controlled room kept at 15 – 20 °C. Canning takes place at the beginning of July and for five weeks thereafter. Ten days prior to the premiere the final product is distributed to wholesalers.[citation needed] The fermentation of the fish depends on a lactic acid enzyme in the spine that is activated if the conditions are right (temperature and brine concentration). The low temperatures in Northern Sweden is one of the parameters that affects the character of the final product.[citation needed]

Fermentation continues in the can which causes the can to bulge noticeably. Prior to modern canning methods, surströmming was sold in wooden barrels, and was only consumed locally. As even the smallest one litre kegs could leak, surströmming was bought directly from the producers in small quantities for immediate consumption.[7]

Half a year to a year later gases have built up sufficiently for the once flat tops of the cylindrical tins to bulge into a more rounded shape. These unusual containers of surströmming can be found today in supermarkets all over Sweden. However, certain airlines have banned the tins on their flights, considering the pressurised containers to be potentially dangerous (see also below).[8] Species of Haloanaerobium bacteria are responsible for the in-can ripening. These bacteria produce carbon dioxide and a number of compounds that account for the unique odour: pungent (propionic acid), rotten-egg (hydrogen sulfide), rancid-butter (butyric acid), and vinegary (acetic acid).[9]

Surströmming is often eaten with a kind of bread known as tunnbröd ("thin bread"). This thin, either soft or crispy bread (not to be confused with crispbread) comes in big square sheets when soft or as rounds of almost a metre in diameter when crisp.[10]

The custom in The High Coast (Höga Kusten), the area of northern Sweden where this tradition originates, is to make a sandwich, commonly known as a "surströmmingsklämma", using two pieces of the hard and crispy kind of tunnbröd with butter, boiled and sliced or mashed potatoes (often mandelpotatis or almond potatoes) topped with fillets of the fish together with finely diced onions. It is also eaten on the plate with the above ingredients. To balance the strong flavour of the fish, Västerbotten cheese is sometimes eaten with it.[11]

In the southern part of Sweden, it is customary to use a variety of condiments such as diced onion, gräddfil (fat fermented milk/sour cream similar to smetana) or crème fraîche, chives and sometimes even tomato and chopped dill.[12]

The surströmming sandwich is usually served with snaps and light beers like pilsner or lager. Other drinks of choice are svagdricka, (lit. "weak drink", a Swedish low alcoholic dark malt beverage brewed since the Middle Ages, a small beer slightly similar to porter),[13] water or cold milk. However, exactly what to drink or not to drink to surströmming is highly disputed among connoisseurs. Some claim that cold milk is the right and only choice while others refer to svagdricka as the most traditional drink. Surströmming is usually served as the focus of a traditional festivity, a "surströmmingsskiva" (surströmming party).

Many people do not care for surströmming, and it is generally considered to be an acquired taste.[14] It is a food which is subject to strong passions, as is lutefisk.

On June 4, 2005, the first surströmming museum in the world was opened in Skeppsmalen,[15] 30 km (19 mi) north of Örnsköldsvik, a town at the northern end of the High Coast.[16] The name of the museum is "Fiskevistet" (translated to The Fish Visit).

Surströmming today contains higher levels of dioxins and PCBs than the permitted levels for fish in the EU; Sweden was granted exceptions to these rules from 2002 to 2011, and a renewal of the exceptions was then applied for. Producers have said that if the application is denied they will only be allowed to use herring less than 17 centimeters long, which contain lower levels, which will affect the availability of herring.[18]

In April 2006, several major airlines (such as Air France and British Airways) banned the fish, claiming that the pressurised cans of fish are potentially explosive. The sale of the fish was subsequently discontinued in Stockholm's international airport. Those who produce the fish have called the airlines' decision "culturally illiterate", claiming that it is a "myth that the tinned fish can explode".[14]

In 1981, a German landlord evicted a tenant without notice after the tenant spread surströmming brine in the apartment building's stairwell. When the landlord was taken to court, the court ruled that the termination was justified when the landlord's party demonstrated their case by opening a can inside the courtroom. The court concluded that it "had convinced itself that the disgusting smell of the fish brine far exceeded the degree that fellow-tenants in the building could be expected to tolerate".[19]